Radiometric measuring devices are usually applied when conventional measuring devices cannot be used due to especially rough conditions at the measuring location. Existing very frequently at the measuring location are e.g. extremely high temperatures and pressures or chemically and/or mechanically very aggressive environmental influences, which make the use of other measuring methods impossible.
Such devices typically include a radioactive radiator, which during operation sends radioactive radiation through the container, and, arranged on a side of the container lying opposite the radiator, a detector, which serves to receive a measured variable dependent, radiation intensity penetrating through the container and to convert such into an electrical signal.
In radiometric measurements technology, a radioactive radiator, e.g. a Co60 or Cs137 preparation, is placed in a radiation protection container at a measuring location, e.g. a container containing a fill substance. The container containing the fill substance can be e.g. a tank, a vat, a pipe or tube, a conveyor belt or any other form of containment.
The radiation protection container has a window, through which radiation emitted from the radiator positioned at a measuring location leaves the radiation protection container for the fill substance container.
Usually, the window is sized to provide a radiation direction such that the radiation penetrates that region of the container that needs to be registered for the measuring. On the oppositely lying side of the fill substance container, the emerging radiation intensity, changed by the fill substance at its fill level, respectively by a change in the density of the fill substance, is registered quantitatively with a detector. The emerging radiation intensity depends on the geometric arrangement and on the absorption. The latter in the case of fill level measurement is dependent on the amount and density of the fill substance in the container located in the path of the radiation. As a result, the emerging radiation intensity is a measure for the current fill level, respectively the current density, of the fill substance in the container.
Currently usually applied as a detector are scintillation detectors having a solid, rigid, scintillation rod, at whose one end a photoelectric transducer, e.g. a photomultiplier, is arranged. The scintillation rod is composed of a special synthetic material, such as e.g. polystyrene (PS) or polyvinyl toluene (PVT), which is optically very pure. Gamma radiation leads to the occurrence of light flashes in the scintillation material. The light of these light flashes is registered by the photomultiplier and converted into electrical pulses. Connected to the photomultiplier is a measuring device electronics, which, based on the electrical pulses, determines a pulse rate, with which the pulses occur. The pulse rate depends on the radiation intensity arriving at the photoelectric transducer and, thus, is a measure for the measured variable to be determined.
Scintillation rods are obtainable today in lengths of about 0.4 m up to 2 m. If a length of 2 m is not sufficient to cover the region to be measured, radiometric measuring devices comprising two or more detectors can be provided, each of which covers a portion of the region to be measured.
An example of this is presented in German Patent DE 10 2004 007 680 A1. The radiometric measuring device described there provides that detector is equipped with a scintillation rod and a photomultiplier connected terminally thereto. Each detector produces a pulse rate corresponding to the radiation intensity striking thereon, and, based on the pulse rates of the individual detectors, a sum signal is derived, which corresponds to the total radiation intensity striking the detectors over the measuring range metrologically registered by the detectors.
This solution has the advantage that the individual detectors can be flexibly arranged and, thus, can be matched to the spatial conditions at the location of use, especially to the container geometry. Disadvantageous, however, is that each detector requires its own photoelectric transducer.
An alternative solution is known from German Gebrauchsmuster (utility model) DE 201 03 881 U1. Such describes a scintillation detector for a radiometric measuring device for measuring a fill level of a fill substance located in a container, comprising:                two or more scintillators arranged in a series relative to one another for converting thereon falling, radioactive radiation into light flashes, whose light propagates in the respective scintillator toward its ends;        optical coupling elements arranged between the scintillators for establishing light transmitting connections between adjoining pairs of scintillators; and,        connected to an end of the series, a photoelectric transducer, which converts light occurring in the series into an electrical signal corresponding to a radiation intensity striking the scintillators.        
The scintillators are placed in a protective tube constructed of segments.
In a first variant described in the above-noted DE 201 03 881 U1, the scintillators are straight rods, which are arranged end to end in a straight line with interpositioning of a material effecting optical coupling, especially a material in the form of silicone disks or silicone oil. In such case, there arises the problem that the different thermal expansions of the scintillators and the segments of the protective tube must be absorbed, respectively cancelled, in order to prevent a degrading of the optical coupling between the aligned scintillators. This is effected in the described variant by a spring, which presses the scintillators toward one another and the straight rod composed of the scintillators, as a whole, toward the photoelectric transducer.
Additionally, a second variant is described, in the case of which there is constructed from the individual scintillators a sectionally angled, total scintillator. For this, wedge shaped, intermediate pieces are inserted between individual scintillators. Described examples of the intermediate pieces are a wedge shaped silicone disk and a wedge shaped light conductor. In such case, the end faces of the light conductors are optically coupled via silicone oil to the end faces of the adjoining scintillators.
Also here, there arises naturally the problem of the different thermal expansions of the scintillators and the segments of the protective tube, a problem that, in spite of the sectionally angled shape, must be solved, in order to assure the optical coupling, especially when silicone oil is being used.
Moreover, there arises here the problem that, at the wedge shaped intermediate pieces, due to the basically straight line light propagation, a certain part of the light to be transmitted to the adjoining scintillators escapes to the exterior. A part of this light escape arises due to the directional change directly outside the intermediate piece. A further part is lost, because, after the directional change through the intermediate piece, the outer surfaces of the following scintillator are no longer at the angle for total reflection, and, thus, light escapes to the exterior from the following scintillator. The lost light is greater, the more strongly the angling caused by the particular intermediate pieces. This lost light does not get to the photoelectric transducer and, thus, is not registered. Light losses act directly disadvantageously on the measuring sensitivity of the measuring device. An as high as possible light efficiency is, however, exactly especially important for these detectors, in the case of which light travels a greater path length through a number of individual scintillators arranged in series. Accordingly, here, both the angle sizes and also the number of the angles are limited. Correspondingly, also the options for matching the spatial conditions at the location of use, for instance round container shapes, are greatly limited.